Let’s be fair to Minty, she has commandeered the car to go and pick you up at the cart stop because while she doesn’t understand your need to return it you are powerful and valuable to her (for the time being at least).
Edit: Also Astarion is mildly offended that you brought up the topic of returning the cart rather than leaving it in the parking lot angled down hill/wind to the most expensive car in the lot.